Technical Abstract:
The need for dietary Trp and its effect on stress of broiler chicks from 0 to 20 d of age was evaluated. Ross x Ross 508 male chicks were used in two studies, a diet-validation and a dose-response study. The first study compared the titration diet (corn-soybean meal-gelatin byproduct based diet) against two corn-soybean meal control diets (marginal and high in dietary lysine). Day-old chicks were randomly distributed across a closed-curtain sided house (24 floor pens; 13 chicks/pen), and fed the experimental diets. Growth and feed intake were determined at 20 d of age. The control diets had higher BW (P < 0.001) and feed intake (P < 0.001) than the titration diet; however, feed conversion of the chicks was similar among treatments. A second study estimated the Trp needs for growth of chicks from 0 to 20 d of age. The titration diet was fed to chicks (49 floor pens, 13 chcks/pen; 7 replicates/trt) providing 0.13% total Trp to which 0.02% increments of L-Trp were supplemented to 0.25% of diet at the expense of a filler. Using regression analysis (95% of minimum or maximum response), it was determined that chicks optimized BW, feed intake and feed conversion at 0.21, 0.20, and 0.22% dietary Trp, respectively. Blood plasma free Trp displayed a sigmoidal response, in agreement with live performance needs. Plasma cholesterol, heterocyte/lymphocyte ratio, high/low density lipoprotein ratio and corticosterone were unaffected by dietary Trp, suggesting that no stress effect was imposed with Trp deficiency. Plasma glucose increased in a linear manner with dietary Trp, perhaps as a result of gluconeogenesis rather than physiological stress. Present results are in agreement with current NRC (1994) recommendations of 0.20% total Trp.